The wood used in paper making consists essentially of cellulose and hemicellulose fibers bound together by a polymeric material called lignin. A goal of the chemically-based wood pulping processes is to separate the cellulose and hemicellulose from the lignin by degrading the lignin into lower molecular weight species that are soluble in water. In a typical pulping process, wood is added to water that contains sodium carbonate, sodium bicarbonate or sodium hydroxide, and the resulting mixture is heated under pressure to a temperature in the range of 140.degree.-180.degree. C. While the wood lignin may be broken down by reaction with the alkali alone, the process is slow and also results in excessive degradation of the cellulose and hemicellulose which are the desired products. In order to shorten the processing time or lower the processing temperature or both, pulp manufacturers use accelerators such as sodium sulfite or sodium sulfide in the alkali solution to aid in processing.
Sulfur based accelerators reduce the processing time and produce a product superior to that produced by alkali alone, but these additives also introduce additional problems. For example, for the sulfide (kraft) process, malodorous air emissions may occur, and expensive chemical recovery equipment is needed in order to reduce chemical costs. While the sulfur containing systems do present several difficulties to the paper making industry, they are nonetheless the standard process used because they offer the overall greatest flexibility, low chemical costs and strongest paper. For example, the lignin containing spent pulping liquor is burned to produce heat, and the sulfur and caustic are recovered from the furnace and recycled.
An alternative to the use of sulfur accelerators was described by Holton in U.S. Pat. No. 4,012,280 in which the addition and use of quinone compounds such as naphthoquinone, anthraquinone (AQ), phenanthrenequinone, anthrone and their ring substituted derivatives are claimed as accelerators in the wood pulping process. AQ is used at &lt;0.1% levels to improve pulping productivity and lower environmental impacts by (a) increasing pulping rates and product yields, (b) reducing chemical recovery bottlenecks, and (c) providing greater removal of lignin, meaning less organic bleaching by-products. It can be used alone (soda/AQ) or with a kraft process; in the latter case, the sulfur content of the liquor can be reduced and less odorous emissions occur.
While AQ is effective as a catalyst for improving pulping productivity and lowering environmental impacts, the commercial use of AQ is restricted due to its relatively high cost. Even so, kraft/AQ pulping is gaining in popularity, largely because of recent AQ cost reductions and industry's desire to extend production without capital expenditures.
One approach to bringing down the cost of AQ is to prepare a mixture of fused ring quinone type compounds from lignin and lignin derived substances. For example, U.S. Pat. Nos. 5,002,634 and 5,049,236 describe processes whereby lignocellulosics, lignin and lignin derived compounds, such as may be found in black liquor obtained by pulping wood, can be reacted through the use of several oxidation techniques to form a mixture of quinone compounds containing benzoquinones, naphthoquinones, and anthraquinones which can be added to the pulping process to accelerate the lignin degradation process. The economics for the preparation of AQ from lignin is hampered by the overall low yield, typically not more than about 5% yield.
More traditional methods for synthesizing AQ typically involve the reaction of naphthaquinone with 1,3-butadiene to yield tetrahydroanthraquinone. The tetrahydroanthraquinone is further oxidized to form AQ. Other procedures involve oxidation of anthracene and condensation of phthalic anhydride with benzene. However, these synthesis are not well adapted to preparing inexpensive substituted AQ or for allowing easy introduction of substituents into the AQ structure during synthesis.
It is an object of the present invention to provide an efficient method for the synthesis of octahydroanthraquinones and anthraquinones which allows for the introduction of substituents.
It is another object of the invention to provide a method for the delignification of lignocellulosic material using substituted octahydroanthraquinones and substituted anthraquinones as pulping catalysts.
Other objects, advantages, features and characteristics of the present invention will become more apparent upon consideration of the following description and the appended claims.